12-Molybdophosphoric Acid, H₇[P(Mo₂O₇)₆]

12-Molybdophosphoric Acid, H₇[P(Mo₂O₇)₆].xH₂O, which is identical with the phosphomolybdic acid described above, may be obtained by heating the ammonium salt with aqua regia and recrystallising from water containing a little nitric acid; by dissolving the yellow hydrate, MoO₃.2H₂O, in a hot solution of phosphoric acid and allowing to crystallise; or by adding an excess of hydrochloric acid to a solution of an alkali molybdate and phosphoric acid and then extracting the molybdophosphoric acid with ether.

It is readily soluble in water, and on evaporation the solution deposits deep yellow octahedral crystals of the 28-hydrate H₇[P(Mo₂O₇)₆].28H₂O. On heating this hydrate it begins to melt at about 78° C., and a clear liquid is obtained at 98° C., from which the 12-hydrate H₇[P(Mo₂O₇)₆].12H₂O may be separated in the form of minute crystals by carefully cooling to about 90° C. If the acid is crystallised from concentrated nitric acid solution, small rhombic crystals of a 22-hydrate, H₇[P(Mo₂O₇)₆].22H₂O, are obtained. Many other hydrates are described by earlier investigators, but there is little doubt that such were either identical with those described above or were not pure compounds.

Salts of this acid have been prepared by treating a solution of an alkali molybdate with a small quantity of phosphoric acid; by adding a mineral acid, preferably concentrated nitric acid, to a solution containing a phosphate and a large excess of molybdate; or by boiling an aqueous solution of an alkali phosphate saturated with molybdic anhydride. The ammonium and potassium salts separate out as deep yellow micro-crystals, but the sodium salt is soluble, and is obtained by concentrating the solution when it forms yellow efflorescent rhombohedra. These salts have the approximate composition 3R^•₂O.P₂O₅.24MoO₃.xH₂O, where R^• = NH₄, K, or Na. Corresponding salts of barium, cobalt, nickel, manganese, and iron have also been prepared by boiling aqueous suspensions of the metallic phosphates with molybdic anhydride.

More exact information as to the nature of these compounds was obtained by a study of the guanidinium salts. The tribasic salt, corresponding with those described above, is obtained by dissolving 12 molecules of molybdic anhydride in a boiling solution of guanidinium carbonate (containing 12 molecules), adding 1 molecule of phosphoric acid, and acidifying strongly with hydrochloric acid. The salt has the composition (CH₅N₃)₃H₇[P(Mo₂O₇)₆].10H₂O, and measurements of its electrical conductivity show it to be an acid salt. If a 10 per cent, solution of guanidinium carbonate is gradually mixed with a solution of 12-molybdophosphoric acid until the deep yellow precipitate first formed redissolves in the warm solution, greenish-yellow crystals of the heptabasic salt, (CH₅N₃)₇H₇[P(Mo₂O₇)₆].8H₂O, separate on cooling. Conductivity measurements prove this to be a normal salt.

The sodium salt described above has been shown to be an acid salt of composition Na₃H₄[P(Mo₂O₇)₃].19H₂O. The heptabasic silver and mercurous salts formed by precipitation from solutions of 12-molybdophosphoric acid by means of the metal nitrates are very probably the normal salts of the acid.

The existence of 11- and 10-molybdophosphoric acids and their salts, although many such have been described, is not established.